Mass spectrometry with selective ion filtration by digital thresholding

ABSTRACT

The methods described herein generally relate to characterization of large analytes, such as biomolecules, by molecular mass analysis. Specifically, the methods are directed to molecular mass analysis of singly— or multiply-charged ions by selective ion filtering carried out by a digital thresholding process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 12/694,131 filed Jan. 26, 2010, which is a continuation of U.S. application Ser. No. 11/136,134 filed May 24, 2005, now U.S. Pat. No. 7,714,275 issued May 11, 2010, which claims the benefit of priority of U.S. Provisional Application Ser. No. 60/574,042 filed May 24, 2004, the entire disclosure of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to the field of detection and characterization of large analytes, such as biomolecules, by molecular mass analysis.

BACKGROUND OF THE INVENTION

Mass spectrometry has been used for many decades in the characterization of small organic molecules. The technique typically involves the ionization of molecules in the sample to form molecular ions by subjecting the sample to an electron beam at a very low pressure. The molecular ions are then focused and accelerated by an electric field into a magnetic field or quadrupole. The ions are separated in the magnetic field or quadrupole according to the ratio of the mass of the ion m to the charge on the ion z (m/z). After passing through the field, the ions impinge upon a detector which determines the intensity of the ion beam and the m/z ratio, and these data are used to create the mass spectrum of the sample.

With the increasing interest in larger molecules, especially biomolecules such as nucleic acids and proteins, new techniques in the field of mass spectrometry are continually being developed to characterize these molecules.

In recent years the performance of commercially available mass spectrometers has seen significant improvement due, in part, to the availability of improved core components including more stable power supplies, faster digitizers, and more sophisticated fabrication methods for ion optic elements. Particularly noteworthy are the newest generation ESI-TOF mass spectrometers which, from several vendors in a variety of configurations, are now routinely yielding the types of mass measurement accuracy and mass resolution previously attainable only on high end sector or Fourier transform ion cyclotron resonance (FTICR)-based platforms. As such, the use of such bench top instruments by the bioanalytical community continues to expand as these instruments are increasingly being made available to scientists and technicians with a broad range of analytical needs. Accordingly, a number of increasingly sophisticated automation schemes are emerging, many incorporating some form of liquid chromatography (LC) as an on-line sample purification step to support high throughput QC or drug screening activities. While there are a number of applications in which some form of LC is a requisite step that facilitates the analysis of very complex mixtures, it is also used frequently as a generic desalting/purification protocol to prepare relatively pure analyte fractions for MS analysis.

Low molecular weight chemical noise is often the limiting factor in overall MS performance as the presence of high levels of low molecular weight components, such as polymers and buffer constituents, can drastically limit the spectral dynamic range and adversely affect mass accuracy. While LC is often used to reduce the adverse affects of such backgrounds, constraints on sample throughput and issues associated with solvent usage/disposal must be considered as part of the laboratory work flow. Additionally, LC is often used as a purification step (as opposed to a separation step) to render analytes amenable to MS analysis. Consequently, there is an increasing need for simple methods to reduce the chemical noise floor and render less than “pristine” samples amenable to mass spectrometric analysis.

The present invention satisfies this need, as well as others, by providing systems and methods for digital filtration of mass spectral signals arising from singly-charged low molecular weight components such as solution additives and matrix modifiers without significantly altering the mass spectral signals of larger analytes such as biomolecules.

SUMMARY OF THE INVENTION

The present invention is directed to methods of identifying a multiply-charged ion. A mass spectrometer that comprises the following components is provided: (i) an ion detector, (ii) a digitizer that converts an analog signal to a digital signal, (iii) an analog signal transfer means for transferring an analog signal from the detector to the digitizer, and (iv) a digital threshold filter which is in digital data communication with the digitizer. A digital signal threshold can be set at the digital threshold filter and, in response to a digital signal input from the digitizer, the digital threshold filter independently outputs a digital signal to a data file only if the digital signal input is greater than the specified digital signal threshold. The continuing step of the method is then effected by specifying a digital signal threshold such that, upon a mass spectrometer measurement of the multiply-charged ion, the filtered digital signal output to the data file originates from the detection of the multiply-charged ion and excludes digital signal output from analog signals arising from singly-charged ions.

The present invention is also directed to methods for determining the molecular mass of a plurality of analytes in a mixture. A mass spectrometer that comprises the following components is provided: (i) an ion detector, (ii) a digitizer that converts an analog signal to a digital signal, (iii) an analog signal transfer means for transferring an analog signal from the detector to the digitizer, and (iv) a plurality of digital threshold filters, each in digital data communication with the digitizer. A digital signal threshold can be independently set at any of the plurality of digital threshold filters, each of which is in digital data communication with the digitizer and, in response to a digital signal input from the digitizer, independently outputs a digital signal to a corresponding data file only if the digital signal input is greater than the specified digital signal threshold. The continuing steps of the method are then effected by specifying a unique digital signal threshold at some members of the plurality of digital threshold filters, making a mass spectrometer measurement of the mixture, wherein each unique digital signal threshold differentially filters digital signals arising from the plurality of analytes and produces a unique digital signal output to each corresponding data file. The measurement results in storage of a plurality of data files. In the final step, each of the plurality of data files is analyzed and the molecular mass of at least one member of the plurality of analytes is contained in each of the plurality of data files.

The present invention is also directed to methods for calibrating a mass spectrum of an analyte. A mass spectrometer that comprises the following components is provided: (i) an ion detector, (ii) a digitizer that converts an analog signal to a digital signal, (iii) an analog signal transfer means for transferring an analog signal from the detector to the digitizer, and (iv) a plurality of digital threshold filters, each in digital data communication with the digitizer. A digital signal threshold can be independently set at any of the plurality of digital threshold filters, each of which is in digital data communication with the digitizer and, in response to a digital signal input from the digitizer, independently outputs a digital signal to a corresponding data file only if the digital signal input is greater than the specified digital signal threshold. The continuing steps of the method are then effected by specifying a first unique digital signal threshold at one digital threshold filter such that digital signal output to a first data file has signals from both the analyte and a calibrant ion and then specifying a second unique digital signal threshold at another digital threshold filter such that the digital signal output to a second data file has signals from the analyte but not the calibrant. The second data file is subtracted from the first data file to obtain a calibration file which is then used to calibrate the mass spectrum.

The present invention is also directed to a system comprised of a mass spectrometer that comprises the following components: (i) an ion detector, (ii) a digitizer that converts an analog signal to a digital signal, (iii) an analog signal transfer means for transferring an analog signal from the detector to the digitizer, and (iv) a plurality of digital threshold filters for setting a digital signal threshold which are each in digital data communication with the digitizer and in response to a digital signal input from the digitizer independently outputting a digital signal to a corresponding data file only if the digital signal input is greater than the specified digital signal threshold. The system has a plurality of data files and a plurality of parallel digital signal output transferring means, each of which is in digital data communication with one of the plurality of digital threshold filters and a corresponding data file from the plurality of data files.

BRIEF DISCUSSION OF THE DRAWINGS

FIG. 1 shows the detector response intensity as a function of digital signal threshold value (in this case indicated by cutoff voltage) of analyte ions having similar m/z ratios but differing in molecular weights. Circles: 140-mer oligonucleotide (m/z=1232.9), squares: 70-mer oligonucleotide (m/z=1199), diamonds: 38-mer oligonucleotide (m/z=1174.7), crosses: 12-mer oligonucleotide (m/z=1233) and triangles: polypropylene glycol (PPG−m/z=1236).

FIG. 2 is a schematic representation of the effects of specifying digital signal thresholds on mass spectra. FIG. 2 a depicts the raw digitizer (ADC, analog digital converter) output from a theoretical single scan containing a singly-charged ion (ion1) which strikes the detector at T1 and a large multiply-charged ion (ion2) which strikes the detector at T2. FIGS. 2 b and 2 c indicate a spectrum with a high and low digital signal threshold respectively. FIG. 2 d indicates a spectrum without a digital signal threshold and detector “white noise” is visible in the spectrum.

FIG. 3 displays mass spectra of a PCR product. FIG. 3 a is an ESI-TOF mass spectra of a 140-mer PCR product acquired at a normal (3 mV) digital threshold setting. The sample contains a contaminating amount of polypropylene glycol (PPG) relatively high levels of singly charged peptides (which serve as internal mass standards). Peaks labeled with “x” indicate signals from the PPG and “c” represents signals from the peptide mass standards. FIG. 3 b is an ESI-TOF spectrum of the same sample of PCR product obtained at a digital threshold setting of 15 mV. Contaminants and mass standards have been filtered out of the spectrum.

FIG. 4 is an expanded region of the ESI-TOF spectra from FIG. 3 in which the relatively low abundance high charge states of the PCR amplicon are detected. The effective signal to noise of the spectrum in FIG. 4 a is defined by the signal to chemical noise ratio, while the effective signal to noise of the spectrum in FIG. 4 b is defined by the signal to electronic noise ratio.

FIG. 5 exhibits ESI-TOF spectra of a solution containing approximately 0.5 nM PCR product in the presence of 500 nM PPG was characterized at low (FIG. 5 a) and high (FIG. 5 b) threshold settings As shown in the inset, the top spectrum is also inundated with other chemical noise components and the peak-at-every-mass background precludes the detection of the low level PCR products. When the digital signal threshold is set such that signals from singly charged species are not detected, a distinct signature for the low level amplicon is detected.

FIG. 6 indicates two overlapping peaks of a 140-mer oligonucleotide and of a 12-mer oligonucleotide which can be resolved through acquisition of data with different digital signal thresholds. The top spectrum was obtained with a digital signal threshold setting of 7 mV. The middle spectrum was obtained with a digital signal threshold setting of 11 mV. The bottom spectrum was obtained by subtraction of the middle spectrum from the top spectrum to obtain a clean isotopically resolved spectrum of the 12-mer oligonucleotide.

FIG. 7 shows the typical digitizer configuration (FIG. 7 a) with a single threshold setting compared to a digitizer which allows multiple threshold settings to be applied simultaneously to data stream coming from the TOF digitizer (FIG. 7 b).

FIG. 8 shows mass spectra of carbonic anhydrase in the presence of 0.001% SDS and P/I buffer. The protein-derived signals of the spectrum obtained with a 1 mV digital signal threshold setting (FIG. 8 a) are subject to considerable interference from the detergent and buffer components. In contrast, FIG. 8 b indicates that the interfering components are rendered “invisible” by specifying a digital threshold setting of 11 mV.

DESCRIPTION OF EMBODIMENTS

In some embodiments of the present invention, the mass spectrometer system comprises the following components: (i) an ion detector, (ii) a digitizer that converts an analog signal to a digital signal, (iii) an analog signal transfer means for transferring an analog signal from the detector to the digitizer, and (iv) a plurality of digital threshold filters for setting a digital signal threshold which are each in digital data communication with the digitizer and in response to a digital signal input from the digitizer independently outputting a digital signal to a corresponding data file only if the digital signal input is greater than the specified digital signal threshold. In some embodiments, the analog and digital signals are voltage signals and the analog to digital converter (ADC) converts the analog voltage signal to a digital voltage signal. In some embodiments, a plurality of mass spectrometer measurements are made and the resulting plurality of data files are co-added.

In other embodiments, the mass spectrometer system comprises a (iv) single digital threshold filter instead of a plurality of digital threshold filters. The single digital threshold filter is in digital data communication with the digitizer and a corresponding data file.

In some embodiments, the mass spectrometer is a time-of-flight mass spectrometer, a quadrupole time-of-flight mass spectrometer, a linear quadrupole mass spectrometer, a linear trap mass spectrometer, an electric/magnetic sector mass spectrometer or a quadrupole ion trap mass spectrometer. In some embodiments, ions are produced by electrospray ionization (ESI).

In some embodiments, the multiply-charged analyte is a biomolecule such as, for example, a nucleic acid, a protein, a carbohydrate or a lipid. Examples of nucleic acids include, but are not limited to, RNA constructs used to screen small molecules for drug discovery and amplification products such as PCR products which can be used for genetic analyses. In some embodiments, the multiply-charged analyte is of a molecular weight of 5-500 kDa, 25-250 kDa, or 50-100 kDa.

In some embodiments, the method allows for ESI-TOF characterization of biomolecules in the presence of biomolecule stabilizing agents or matrix modifiers used in online separation techniques. Stabilizing agents include, but are not limited to, buffer salts such as phosphates for example, ampholytes, glycerol, polyethylene glycol, polypropylene glycol, reducing agents, detergents, and the like. Matrix modifiers may be any type of additive used to effect a solution matrix property advantageous to an analytical separation and may include, but are not limited to, ampholytes, detergents and buffer salts such as phosphates for example.

In some embodiments, the biomolecule stabilizing agents or matrix modifiers are singly-charged when detected by the mass spectrometer. In other embodiments, the biomolecule stabilizing agents or matrix modifiers have one or two charges.

In some embodiments, when a plurality of digital signal threshold filters are employed in the mass spectrometer system, a plurality of unique digital signal thresholds are specified in order to obtain parallel differentially filtered data streams which are stored in corresponding data files. In some embodiments, any member of the data files may be subtracted from any of the other data files to obtain a more accurate representation of a given analyte signal. These embodiments may be used to obtain a more accurate mass spectrum of a calibrant ion, or any other lower molecular weight contaminating ion by subtracting out an overlapping signal from an ion having a similar m/z but with a larger molecular mass.

In some embodiments, the methods described herein which employ multiple differentially thresholded data streams may be used in multiplexed data acquisition of a plurality of ions such as those obtained from chemical, protease or restriction digestion of proteins or nucleic acids.

In some embodiments, the methods described herein may be used to reduce the burden of level of purification of large molecular weight or multiply charged analytes such as biomolecules, for example, from stabilizing agents or matrix modifiers.

EXAMPLES Example 1 ESI-TOF Mass Spectrometry Conditions

A Bruker Daltonics (Billerica, Mass.) MicroTOF ESI time-of-flight (TOF) mass spectrometer was used in this work. Ions from the ESI source undergo orthogonal ion extraction and are focused in a reflectron prior to detection. Ions are formed in the standard MicroTOF ESI source which is equipped with an off-axis sprayer and glass capillary. For operation in the negative ion mode, the atmospheric pressure end of the glass capillary is biased at 6000 V relative to the ESI needle during data acquisition. A counter-current flow of dry N2 is employed to assist in the desolvation process. External ion accumulation is employed to improve ionization duty cycle during data acquisition. Each ESI-TOF spectrum is comprised of 75,000 data points digitized over 75 μs. All aspects of data acquisition were controlled by the Bruker MicroTOF software package. Post processing of data was also performed using the standard Bruker software.

Example 2 PCR Conditions and Purification of Amplification Products

All PCR reactions were assembled in 50 μL reaction volumes in a 96 well microtiter plate format using a Packard MPII liquid handling robotic platform and M. J. Dyad thermocyclers (MJ research, Waltham, Mass.). The PCR reaction mix consists of 4 units of Amplitaq Gold, 1× buffer II (Applied Biosystems, Foster City, Calif.), 1.5 mM MgCl₂, 0.4M betaine, 800 μM dNTP mix and 250 nM of primer. The following PCR conditions were used: 95° C. for 10 min followed by 50 cycles of 95° C. for 30 sec, 50° C. for 30 sec, and 72° C. for 30 sec.

PCR products were purified using the protocols disclosed and claimed in U.S. patent application Ser. No. 10/943,344 which is commonly owned and incorporated herein by reference in entirety.

Example 3 Investigation of Detection Efficiency of Large Oligonucleotide Ions

In an attempt to optimize detection efficiency of large oligonucleotide ions, and to better understand the relationship between ion arrival statistics and mass accuracy, a detailed systematic study was designed to investigate detector response as a function of molecular weight, m/z, and charge state at the individual ion level.

In time of flight mass spectrometry ions are separated based on differences in their velocity as they traverse the flight tube. As ions strike the detector, their arrival times are recorded and subsequently converted to m/z based on the specific configuration of the spectrometer (length of flight path, accelerating voltage, geometry, etc.). It is generally accepted that for singly charged species, detector response is inversely proportional to molecular weight (velocity) and, for example in the case of MALDI, higher molecular weight species induce a smaller detection signal than lower molecular weight species. It was suspected that lower charge states (i.e. lower velocity species) induce a smaller signal than do the higher charge states (i.e. high velocity species) under the same accelerating voltages. The reduced response of high molecular weight “slow” ions can be partially ameliorated by the use of post-acceleration methods in which ions are accelerated to very high kinetic energies immediately prior to detection.

During the course of this investigation, it became immediately apparent that ions of the same nominal m/z but different molecular weights induced significantly different detector responses. The heavier, more highly charged ions consistently produced detector responses several times that of their singly charged counterparts at the same m/z. Thus, while in the TOF mass analyzer ions of the same m/z have the same velocity, ions of different molecular weigh do not have the same momentum or kinetic energy and do not induce the same signal on the detector.

This phenomenon is readily illustrated by examining spectral response as a function of the digital threshold employed to acquire mass spectra of species covering a range of molecular weights.

Unlike MALDI of large biomolecules, the multiple charging phenomenon inherent to the ESI process generally produces mass spectra in which the majority of the signals are in the same m/z range. Molecular ions from moderate to large biomolecules (1 kDa to 100 kDa) are generally detected in the 500-2000 m/z range and it is thus not at all uncommon for complex mixtures to yield spectra in which peaks of many different masses are detected at the same m/z. To characterize detector response as a function of molecular weight (charge), solutions containing analytes with molecular weight ratios of 1.0, 3.7, 11.8, 21.5, and 43 were analyzed at a range of digital thresholds. For each series, a single charge at or near m/z 1233 was used to gauge the detector response. The resulting molecular weight isopleths are plotted in FIG. 1. Importantly, at low digital signal thresholds set according to Example 4 (vide infra), the singly charged PPG ions drop in intensity at significantly lower cutoff voltages than do the higher molecular weight (charge) species. For example, at a digital signal threshold cutoff voltage of 9 mV, the signal of the PPG ions at m/z 1233 is attenuated to non-detectable levels while the 43 kDa PCR product at m/z 1233 is still detected at approximately 90% of the initial response. There is a definite trend in cutoff voltages as a function of molecular weight (charge state) suggesting that one can select a digital signal threshold to selectively detect (or not detect) species of interest.

Example 4 Digital Signal Threshold Rationale

Under the acquisition conditions routinely employed to characterize PCR products, individual scans are acquired and co-added at a rate of 75 kHz. Thus for a typical 45 second acquisition, each spectrum is comprised of 660,000 co-added individual scans. In order to reduce the shot/white noise in the co-added spectrum, the MicroTOF electronics allow one to set a digital filter threshold (voltage cutoff) such that white noise from the detector at the single or low-bit ADC count is zeroed out of each scan and only detector responses consistent with ion detection events are passed to the transient summing digitizer data system to be co-added. This concept is shown schematically in FIG. 2. FIG. 2 a depicts the raw ADC output from a theoretical single scan in which a singly charged ion (ion1) strikes the detector at T1 and a large multiply charged ion (ion2) which strikes the detector at time T2. During the time intervals in which neither ion1 nor ion2 are striking the detector the ADC is picking up and digitizing detector noise generally corresponding to 1-5 bits. Because of the fast acquisition rate of the TOF and the finite ion capacity of the source, each scan is typically comprised of relatively few ion detection events and for any given ion channel, it is very unlikely that an ion will be detected in each scan. Thus, co-adding large numbers of unfiltered scans such as those depicted in FIG. 2 d would result in a noise floor that increases linearly with the number of scans and a mass spectrum in which the ultimate dynamic range would be limited by the relatively high electronic noise floor.

To minimize the deleterious effects of co-adding low-bit detector noise, the MicroTOF electronics allow the user to set a cutoff voltage that has the net effect of zeroing-out low level signals that are attributed only to detector noise. As illustrated in FIG. 2 c, this approach, ideally, does not affect the ADC counts for signals consistent with a singly charged ion but digitally filters each scan prior to co-adding, such that detector white noise is not co-added with the same efficiency as detector ion response. As illustrated in FIG. 2 b, this concept can be taken a step further by setting the digital filter threshold such that ADC counts derived from detector noise and singly charged ions striking the detector are zeroed out prior to co-adding. Thus, with the digital threshold set at the level depicted in FIG. 2 b, a singly charged ion striking the detector is “invisible” in the post-filtered ADC output and the net result is a “high pass” molecular weight (charge) filter in which low molecular weight (charge) species are not detected but high molecular weight (charge) species, which tend to be multiply-charged are still detected.

Example 5 Chemical Noise Removal by High Pass Digital Threshold Filtering

A key challenge in the analysis of large biopolymers by ESI-MS is sample purification. Low molecular weight contaminants in biopolymer solutions can have deleterious effects on the quality of ESI-MS spectra and can significantly limit the dynamic range and accuracy of the measurement. In some cases these low molecular weight “contaminants” are actually required additives as components of an on-line separations. Such additives include ampholytes used in capillary isoelectric focusing, phosphates commonly used as components of buffers used in capillary zone electrophoresis, and solution matrix modifiers used to promote micelle formation in micellar electrokinetic chromatography. Similarly, electrospray incompatible additives such as glycerol and polymers (polyethelene glycol, PPG) are often used to stabilize enzymes to be used in biochemical processes. These compounds often make their way through an entire biochemical process and end up in the mass spectrometer. A key example of the latter type of “contaminant” is the presence of high levels of polyethelene glycol and polypropylene glycol polymers in the Taq polymerase used for PCR. While typically only 1-2 μL of Taq are used in each 50 μL PCR reaction, the relatively high concentration of polymer in the presence of the relatively low concentration of PCR products (typically 10-100 nM), coupled with the fact that such polymers are ionized with high efficiency, may cause a significant chemical noise suppression issue.

FIG. 3 a illustrates an example of an ESI-TOF spectrum of a 140-mer PCR product into which a contaminating amount of PPG was spiked along with relatively high levels of singly charged peptides (which serve as internal mass standards). The signal from the charge state envelope of the multiply charged strands of the PCR amplicons is confounded by the presence of the intense signal arising from the low molecular weight species. This spectrum was acquired using a “normal” digital threshold setting in which the detector white noise output from the digitizer is filtered out but the threshold is set low enough to ensure that signals from singly charged ions are captured. This spectrum is exemplary of a common situation in which a large biopolymer is analyzed in the presence of a significant chemical noise background arising from low molecular weight contaminants. As shown, such interferences can adversely affect the mass accuracy of the measurement and result in reduced spectral dynamic range.

In contrast, the ESI-TOF spectrum in FIG. 3 b was acquired on the same spectrometer from the identical solution using the identical ESI source parameters and acquisitions conditions with the important exception that the spectrum in FIG. 3 b was acquired at a cutoff voltage of 15 mV while the spectrum in 3 a was acquired moments earlier at a cutoff voltage of 3 mV. It is clear from these spectra, and the data presented in FIG. 2 that the 15 mV cutoff setting precludes the detection of the singly charged species in the solution yet facilitates the detection of the larger, more highly charged PCR amplicons. It is evident from the spectra in FIG. 3 and the cutoff profiles in FIG. 2 that the intensity of the amplicon peaks are reduced by about 30%; importantly the peaks from the singly charged polymer and calibrants are not present in the spectrum acquired at the higher cutoff voltage and the spectrum in FIG. 3 b has significantly improved signal-to-chemical noise characteristics. It is worthwhile to emphasize that, no other instrument, solution, or data processing parameters were changed between collecting the spectra in FIGS. 3 a and 3 b, the only difference was the digital signal threshold setting.

Indicating the applicability of the method for biomolecules other than nucleic acids, FIG. 8 shows mass spectra of carbonic anhydrase in the presence of 0.001% SDS and 25 mM Piperidine/Imidizole buffer. The protein-derived signals of the spectrum obtained with a 1 mV digital signal threshold setting are subject to considerable interference from the detergent and buffer components. In contrast, FIG. 8 b indicates that the interfering components are rendered “invisible” by specifying a digital threshold setting of 11 mV.

These data indicate that in some high throughput screening and QC applications a less rigorous sample purification protocol might be employed and chemical noise can be removed via the digital filtering approach described above. Importantly, this approach allows ESI-MS analysis of large biomolecules (or noncovalent complexes) from solutions which might otherwise contain too much chemical noise to produce interpretable spectra.

Example 6 Dynamic Range Enhancement by Digital Threshold Filtering

By reducing or eliminating the chemical noise floor in addition to reducing the electronic noise floor, significant improvements in dynamic range and spectral quality are attainable. This concept is demonstrated in FIGS. 4 and 5. Shown in FIG. 4 is an expanded region of the ESI-TOF spectra from FIG. 3 in which the relatively low abundance high charge states of the PCR amplicon are detected. Note that the signals from the (M-43H+)⁴³⁻, (M-42H+)⁴²⁻, and (M-41 H+)⁴¹⁻charge states are barely visible in the unfiltered spectrum (FIG. 4 a) but clearly visible in the filtered spectrum (FIG. 4 b). The effective signal to noise of the spectrum in FIG. 4 a is defined by the signal to chemical noise ratio, while the effective signal to noise of the spectrum in FIG. 4 b is defined by the signal to electronic noise ratio. For example, for the (M-41H+)⁴¹⁻charge state of the amplicon the signal to (chemical) noise in the spectrum acquired at the low cutoff threshold is approximately 2 while the signal to (electronic) noise of the spectrum acquired at the higher cutoff threshold is approximately 12. Additionally, signals from charge states (M-40H+)⁴⁰⁻and (M-39H+)³⁹⁻are not readily discernable from the chemical noise in FIG. 4 a but clearly visible in FIG. 4 b.

The improvement in effective dynamic range afforded by the present invention is further illustrated in FIG. 5 in which a solution containing approximately 0.5 nM PCR product in the presence of 500 nM PPG was characterized at high and low threshold settings. At the normal threshold setting the spectrum is dominated by highly abundant singly charged polymer ions and the very low level PCR products are not observed. As shown in the inset, the top spectrum is also inundated with other chemical noise components and the peak-at-every-mass background precludes the detection of the low level PCR products. When the digital signal threshold is set such that signals from singly charged species are not detected, a distinct signature for the low level amplicon is detected. This attribute has the potential to significantly improve the detection of low concentration biomolecules in solution as it is frequently the presence of low level, ubiquitous, contaminants introduced from buffer impurities, plasticware, and sample handling that define the chemical noise floor of the mass spectra and limit the applicability of ESI-MS to complex biological systems.

In addition to reducing the useful dynamic range of a mass spectrum, chemical noise and low molecular weight contaminants can have adverse affects on accurate mass measurements. As described above, ESI-MS spectra often have overlapping peaks that result from species of different molecular weights but the same m/z. This is particularly problematic for large biopolymer ions which generally produce somewhat congested spectra in which multiple charge states are observed in the 500 to 2000 m/z range. Because low molecular weight species are isotopically resolved and species above about 10 kDa are generally not, it is quite common to see a low molecular weight contaminant peak overlap with and distort an otherwise analytically useful analyte peak. An example of this is shown in FIG. 6 in which the signal from the (M-3H+)³⁻charge state of a 12-mer oligonucleotide is observed at the same m/z as the (M-35H+)³⁵⁻charge state of a much larger 140-mer PCR product. In this case the smaller oligonucleotide is intended to serve as an internal mass standard but, as is illustrated in FIG. 6 and in the mass accuracy data in Table 1, the co-location of these signals is deleterious to both signals. First, at the 7 mV threshold it is not immediately apparent that there are two species at m/z 1233 as peaks from the isotopically resolved 12-mer mask the presence of the larger unresolved amplicon peak. Additionally, the presence of the unresolved amplicon peak distorts the peak shapes and centroids of the isotopically resolved 12-mer peaks such that the mass accuracy is compromised. When the digital threshold is set to 11 mV, the contribution to the peak from the triply charged 12-mer is substantially reduced and the presence of a high molecular weight unresolved peak is apparent. Importantly, because the aggregate signal (i.e. 12-mer and 140-mer) is captured at the 7 mV digital threshold level, and the contribution to the signal from the 140-mer can be measured at a higher digital threshold level (11 mV in this example), the signal from the 12-mer can be derived by subtracting the spectrum acquired at 11 mV from the spectrum acquired at 7 mV. The resulting spectrum exhibits a notably improved distribution (containing 5 peaks) and, perhaps more importantly, the centroided peaks yield a reduced mass measurement error across the distribution. In this example, the average mass measurement error for the five peaks was reduced from 5.9 to 1.5 ppm following the spectral subtraction.

TABLE 1 Calculated Error in m/z Measurements for 7 mV Digital Signal Threshold vs. (7 mv)-(11 mV) Digital Signal Thresholds Digital Signal Threshold Peak Theoretical Measured Error (mV) Number (m/z) (m/z) (ppm) 7 1 1232.5408 1232.5596 −15.2512 7-11 1 1232.5408 1232.5449 −3.3246 7 2 1232.8752 1232.8809 −4.6266 7-11 2 1232.8752 1232.8753 −0.0844 7 3 1233.2095 1233.2005 7.2811 7-11 3 1233.2095 1233.2071 1.9292 7 4 1233.5437 1233.5452 −1.1898 7-11 4 1233.5437 1233.5428 0.7558 7 5 1233.8780 1233.8764 1.2781 7-11 5 1233.8780 1233.8799 −1.5585

In Table 1, 7-11 indicates that a spectrum obtained with a digital signal threshold setting of 11 mV was subtracted from a spectrum obtained with a digital signal threshold setting of 7 mV.

Example 7 Spectral Subtraction and Ion Partitioning: Obtaining “Slices of Ions”

In accordance with the present invention, the experiments illustrated in FIGS. 3-5 illustrate that the digital thresholding method described above allows for the detection of large multiple charged biomolecular ions in such a manner so as to render low molecular weight species “invisible” (based on digital thresholding) while the data presented in FIG. 6 illustrates a method by which low molecular weight species can be analyzed in such a manner so as to make large multiple charged biomolecular ions “invisible” (by digital thresholding and spectral subtraction). The results from the relatively simple subtraction described in Example 6 lay the foundation for more sophisticated digital thresholding schemes in which multiple “slices” of a complex ion population can be analyzed simultaneously with the effective result being a multidimensional detection configuration in which ions are simultaneously measured.

In this work all of the high threshold/low threshold comparisons were made by multiple measurements of the same analyte solution acquired under identical instrument conditions except the digital threshold was varied. This was done out of necessity because, as illustrated in FIG. 7 a, the basic system architecture of the Bruker MicroTOF consists of a single data stream from the detector to the digitizer for which a single threshold level is applied to the data stream prior to co-adding of scans. As sample throughput is a key driver in many laboratories, requiring each sample to be analyzed two (or more) times at different digital thresholds may not be feasible.

In accordance with the present invention and as a means of circumventing this problem, the alternative digitization scheme illustrated in FIG. 7 b indicates that output from the ADC can be split to multiple parallel data streams, each of which is subjected to a different digital threshold. By subtracting spectra acquired at different digital thresholds, one could obtain a mass spectrum for any “slice” of the ion population. This would allow one to perform digital thresholding on a very complex mass spectrum and evaluate a range of molecular weights (charges) independent of other, potentially interfering, ion populations such as for example, a restriction digest of a nucleic acid or a protease digest of a protein. Another example could be a biomolecule such as a nucleic acid or a protein having a non-convalently-bound small molecule.

Having multiple variably-thresholded mass spectra derived from the identical digitization event would guarantee perfect subtraction of spectral features and would eliminate potential artifacts which may arise from spectral drift over the course of acquiring multiple spectra. Importantly, this also means that one could introduce low molecular weight internal mass standards (calibrants) to very accurately calibrate the m/z axis (e.g. the PPG series in FIG. 3 a) but derive accurate mass measurements of biomolecular analytes from peaks that are never “stepped on” by low molecular weight species (e.g. the digitally thresholded spectrum in FIG. 3 b).

Various modifications of the invention, in addition to those described herein, will be apparent to those skilled in the art from the foregoing description. Such modifications are also intended to fall within the scope of the appended claims. Each reference (including, but not limited to, journal articles, U.S. and non-U.S. patents, patent application publications, international patent application publications, gene bank accession numbers, and the like) cited in the present application is incorporated herein by reference in its entirety. 

What is claimed is:
 1. A method of identifying a multiply-charged ion comprising: a) specifying a first and a second digital signal thresholds on a ion identification apparatus, wherein the ion identification apparatus comprises: i) an ion detector; ii) a digitizer for converting an analog signal to a digital signal; iii) an analog signal transfer means for transferring an analog signal from the detector to the digitizer; and iv) a digital threshold filter, for setting a digital signal threshold, in digital data communication with the digitizer, in response to a digital signal input from the digitizer, independently outputting a digital signal to a data file only if the digital signal input is greater than the specified digital signal threshold; wherein upon an ion identification apparatus measurement of the multiply-charged ion, the filtered digital signal output to the data file originates from the detection of the multiply-charged ion and excludes digital signal output from analog signals arising from singly-charged ions; b) obtaining spectrum data of said multiply-charged ion at each of said first and second digital signal thresholds; and c) subtracting said second spectrum data from said first spectrum data to identify said multiply-charged ion as a clean isotopically resolved spectrum.
 2. The method of claim 1 wherein the multiply-charged ion is a biomolecule.
 3. The method of claim 2 wherein the biomolecule is a nucleic acid, peptide, protein, lipid, or carbohydrate.
 4. The method of claim 2 wherein the biomolecule comprises a non-covalently-bound small molecule.
 5. The method of claim 1 wherein the singly-charged ions are biomolecule stabilizer additives.
 6. The method of claim 5 wherein the stabilizer additives are one or more of: ampholytes, polyethylene glycol, glycerol, reducing agents, detergents or buffer salts, or any combination thereof.
 7. The method of claim 1 wherein the analog signal is an analog voltage signal and the digital signal is a digital voltage signal. 